The present invention relates to a press machine that is configured so that the actual bottom dead center position of a slide can be corrected to the target bottom dead center position while controlling the rotation of an adjustment motor using a bottom dead center position correction control section.
It is important for a press machine that the relative distance (die height) between the bottom surface of a slide (upper die) and the upper surface of a bolster (lower die) be maintained constant. This is because a change in die height directly affects the product accuracy.
However, since a change in slide position occurs due to the press structure, thermal deformation (expansion/contraction) of a mechanical component caused by frictional heat, a change in press speed, and the like, it is difficult to continuously perform a press operation while maintaining a constant die height.
Therefore, a bottom dead center position correction control section is generally provided so that the actual bottom dead center position of the slide is corrected to the target bottom dead center position (i.e., the die height is made constant) while controlling the rotation of an adjustment motor by utilizing electrical signals corresponding to a target value (target bottom dead center position data) and a feedback value (bottom dead center position data detected during a press operation).
When the press speed is in a stable state, the amount of change in slide position due to a change in press speed is small. Therefore, even if the die height has been maintained almost constant before starting position correction control, positive position correction control and negative position correction control may be repeated after starting position correction control. Specifically, position correction control is performed in the same manner as in the case where the die height has changed due to a mechanical disturbance. In this case, control tends to become unstable when performing position correction control corresponding to each press cycle. A hunting phenomenon or an out-of-control state is likely to occur as the press speed increases.
Therefore, a related-art bottom dead center position correction control section (see JP-A-60-141399, for example) utilizes a correction unnecessary range (dead zone) while employing a control method that sets the bottom dead center position of the slide within a relatively wide given range in order to reduce a hunting phenomenon. When the bottom dead center position has fallen outside the given range, the bottom dead center position correction control section prompts the worker to take appropriate measures (manual operation) by raising an alarm.
Specifically, the related-art bottom dead center position correction control section performs position correction control corresponding to each press cycle on the assumption that the press speed has reached the set speed (e.g., 60 SPM) (during press operation) (i.e., the press speed is in a stable state (press operation at a constant set speed). In other words, this approach considers position correction control to be unnecessary during acceleration to the set speed from a stationary state or deceleration from the set speed to a stationary state since a product is not produced.
Another approach adjusts a change in die height that occurs during a stationary (stop) period between the preceding press operation and the current press operation. Specifically, since position correction control during a press operation is difficult, a number of devices that are configured to be able to adjust a change in die height before starting a press operation have been proposed (see JP-A-63-299899, JP-A-2-80200, and JP-A-5-8098, for example).
The user of a high-speed press machine (e.g., 200 to 800 SPM or more) generally desires to produce a press product during a rise period from a low speed (e.g., 200 SPM) to the set speed (e.g., 800 SPM) and a fall period from the set speed to a stationary (stop) state from the point of effectively utilizing the press machine. This particularly applies to a large high-speed press machine of which the press speed changes over a long period (e.g., 30 seconds or more).
However, the related-art bottom dead center position correction control section cannot deal with such a demand. Specifically, the related-art bottom dead center position correction control section cannot mechanically, electrically, and controllably perform position correction control on a large high-speed press machine when the press speed is in a changing state.
The press speed of a high-speed press machine is significantly higher than that of a normal press machine (e.g., 60 SPM) when the press speed is in a stable state. Specifically, the cycle time of the high-speed press machine is very short. Therefore, a novel approach is required to enable stable position correction control.